LCZO -- Overland Water Chemistry, Nutrient Fluxes, Rainfall Chemistry, Throughfall Chemistry -- Includes Weekly Rainfall flux -- Bisley -- (1988-2002)

Changes in the quantity and quality of precipitation as it passes through vegetative cover are important components of both hydrologic and nutrient budgets.

Throughfall over any period depends on the balance between precipitation, evaporation and canopy storage (Horton, 1919; Leonard, 1967; Rutter et al., 1972). If the watershed is divided into different vegetation types based on similarity in throughfall and steamflow, the total throughfall over the watershed can be expressed as:

(1) Pg = Sum( T n A n )+ Sum (Sm Dm)

Where Pg = total throughfall reaching the ground, Tn = canopy throughfall from vegetation type n, An = area of vegetation type n, Sm = stemflow from stem type m and Dm = number of stems in type m.

Using eqn. (1) to estimate total watershed throughfall becomes a problem of determining the minimum number of vegetation types necessary to describe the system at the required level of accuracy. In one of our studies, measured throughfall was compared with actual canopy and stem conditions to estimate the percentages of throughfall for different time periods was calculated by weighting the average throughfall and stemflow measured in representative areas of each vegetation type by the total area of that vegetation group.

Measurements reported here were made in two of the Bisley Research Watershed of the U.S. Forest Service. These adjacent watersheds drain 13.0 ha of highly dissected mountainous terrain that range in elevation from 265 to 455 m. Both watersheds are covered by Tabonuco type forests and were selectively logged at various times between 1860 and 1940 (Scatena, 1988).

The dominant tree in the watersheds in the Tabonuco ( Dacryodes excelsa ) which often comprises as much as 35% of the canopy ( Wadsworth, 1970). Structurally the forest has three dominant layers, a discontinuous emergent strata, a continuous upper stratum at 20 m, and an understory layer. Leaves are mesophyllous and often covered with epiphytic growth.

降水在通过植被覆盖层时，其数量与质量的变化是水文和养分收支的重要组成部分。
在任何时间段内，通过降水、蒸发和冠层储存之间的平衡来衡量通过降水（Horton, 1919; Leonard, 1967; Rutter 等人，1972）。如果根据通过降水和蒸汽流相似性将流域划分为不同的植被类型，则整个流域的总通过降水可以表示为：
(1) Pg = Σ(TnAn) + Σ(SmDm)
其中，Pg = 达到地面的总通过降水，Tn = 来自植被类型 n 的冠层通过降水，An = 植被类型 n 的面积，Sm = 来自茎类型 m 的茎流，Dm = 类型 m 的茎数。
使用公式 (1) 来估计总流域通过降水成为确定描述系统所需精确度下必要的最少植被类型数量的问题。在我们的一项研究中，测量到的通过降水与实际的冠层和茎条件进行了比较，以估算不同时间段通过降水百分比。这是通过对每个植被类型的代表性区域的平均通过降水和茎流进行加权，并以该植被组总面积为权重来计算的。
此处报告的测量是在美国林业局的 Bisley 研究流域中的两个进行的。这两个相邻流域汇水面积为 13.0 公顷，地形切割强烈，海拔高度在 265 至 455 米之间。两个流域都覆盖着 Tabonuco 类型森林，并且在 1860 年至 1940 年间进行了选择性采伐（Scatena, 1988）。
在 Tabonuco 流域中的优势树种为 Dacryodes excelsa，它通常占冠层的 35%，（Wadsworth, 1970）。在结构上，森林有三个主要层次：一个不连续的突出层，一个连续的上层在 20 米处，以及一个下木层。叶片为肉质叶，常被附生植物覆盖。